For example, the fastener or the connecting element may be designed in the form of a fitting elongation screw including a head. The fastener may also be designed in the form of a fitting threaded bolt additionally including a fastening thread.
Fasteners and connecting elements, especially fitting elongation screws and fitting threaded bolts, are generally used to connect two or more components or structures. The fasteners are tightened with relatively great preload forces to prevent an opening of the two components in their common contact area under operational conditions. To attain the necessary preload force, such fitting elongation screws are deformed beyond the elastic region of the material into the plastic region. The elastic region is the region of the material in which the material is exclusively deformed elastically. The plastic region is the region in which the material is deformed both elastically and plastically. This elastic/plastic region is entered by tightening the fitting elongation screw. The tightening process usually is either angle-controlled or yield-controlled.
A fastener for connecting two components in the form of a fitting elongation screw is known from European Patent Application No. 0 313 927 A1. The fitting elongation screw includes a head for transmitting torque and a shank including a threaded portion, at least one elongation portion and at least one fitting portion including protrusions and channels. The threaded portion, the elongation portion and the fitting portion are axially spaced apart along the shank. The elongation portion includes protrusions and channels. The protrusions and channels of the fitting portion and of the elongation portion are different, but they all have the design of a thread. The root diameter of the channels of the fitting portion is more than the root diameter of the channels of the elongation portion, preferably at least by 10 percent. In this way, a plastic transverse contraction, meaning a reduction of the diameter, is prevented in the fitting portion during a deformation of the fitting elongation screw into the transelastic region, meaning a tightening of the fitting elongation screw beyond the elastic region into the plastic region. The outer diameter of the fitting portion is more than the outer diameter of the elongation portion, and it is also more than the outer diameter of the threaded portion. This fitting elongation screw may be produced from a shank having a constant diameter along its length by axial displacement of material, and an enlargement of the outer diameter of the protrusions resulting therefrom. The axial width at the outer diameter of the protrusions is relatively small. Additionally, it is even smaller in the fitting portion than in the elongation portion. Consequently, the bearing portion of the fitting portion is very small. Thus, a minimum surface is usable to receive transverse forces in the fitting portion. The design of the elongation portion including spaced apart protrusions and channels reduces the fatigue strength and the dynamic properties of the fastener.
Another fitting elongation screw is known from German Patent Application No. 2 143 784. The function of the fitting portion and the function of the elongation portion are fulfilled by a common portion of the shank of the screw. The shank includes a fitting thread, meaning an inclined arrangement of protrusions and channels in the fitting elongation portion. The root diameter of the fitting elongation portion is less than the root diameter of the connecting portion by 2 to 10 percent. In this way, the elastic elongation and the fatigue strength of the fitting elongation screw is improved. An axial elongation occurs in the region of the fitting elongation portion in combination with a permanent reduction of the outer diameter when the fitting elongation screw is tightened beyond the elastic region. The fitting elongation portion loses the functionality of the fitting portion and the fitting elongation screw is not reusable as desired.
Another fitting elongation screw is known from European Patent No. 0 144 503 B1. The fitting elongation screw includes a shank including a threaded portion and a fitting elongation portion. The fitting elongation portion includes non-inclined protrusions and channels. The outer diameter of the protrusions in the fitting elongation portion is more than the outer diameter in the threaded portion by 0.1 to 0.5 mm. The stressed cross section corresponding to the root diameter of the channels in the fitting elongation portion has the same dimensions as the stressed cross section of the threaded portion. In this way, a fitting elongation screw only being used in the region of an elastic elongation is provided.